The present invention relates to sensing the level of a material and is particularly useful in sensing a level of liquids.
Although the invention is useful in sensing the levels of materials such as liquids in any type of application, the monitoring of the filling of flammable fluids into tank trucks is used herein for purposes of explanation. When tank trucks used for hauling such fluids are filled, it is advantageous from a safety standpoint as well as from a control standpoint to sense the level of the liquid. Overfilling of such a vehicle involves not only waste but a danger of explosion resulting from sparks, static electricity or other forms of ignition. It would, of course, be possible for the operator of the filling pumps to make a visual survey of the liquid level of the tank. However, flammable liquid handling vehicles are often compartmentalized making visual observation of a plurality of tanks impossible or at least very difficult. Also, there may not be the necessary personnel in attendance to supervise the filling operation.
It thus becomes apparent that some form of automatic liquid level control or detection is necessary to supervise the filling operation. The system must also be reliable in view of the grave consequences which could result from a failure of the supervisory system.
Level sensing systems are known in the prior art and involve a number of different types of sensors for monitoring the level of the material or liquid. These types include capacitive sensors, optical sensors, electrical contacts and switches responsive to the level of the material or liquid. As these systems developed, it became apparent that these systems could fail in a mode which would prevent or inhibit a proper responsive by the indicating or control apparatus controlled by the sensor. For example, one such prior art system involves transmitting light from a source through fiber optics to a prism which reflects that light through a second fiber optic element to a photosensitive device as long as the liquid is below the prism. As long as the photosensitive device is receiving light, it operates under the assumption that the liquid is below the prism and will, therefore, maintain the indicator off or the valve which is controlling the filling operation open. Should a break in the second fiber optic element occur, but in a way which allows this broken element to pick up light from another source such as the sun, the photosensitive device will still receive light which will maintain the indicator off or the valve open even though the liquid is above the predetermined level. Thus, the liquid filling will continue and overflow the tank.
The prior art solution to this type of failure mode was to use a pulsating light source and a circuit responsive to the cessation of pulses for providing the proper indication or closing of the valve. In this manner, when the liquid reached the prism, the index of refraction at the prism surface changed and the pulsating light, instead of being reflected by the prismatic surfaces, passed into the liquid. Since the pulses terminated, the indication was given or the valve was closed. If a fiber optic broke and picked up light from another source or if any electronic element in the system failed providing for a continuous signal, again the pulses ceased and the proper indication or closure of the valve was given.
These prior art devices, however, failed to distinguish between the signal pulses of the system and pulses introduced into the system by noise sources such as fluorescent lights, power lines or strong ambient light. These sources of noise can impose signals in the system which are pulsating in nature and which will exist even after the material being sensed has reached the sensor to stop the transmission of the primary sensing signal from the signal source. Thus, the system is made to believe that the material has not reached the sensor and the filling operation will continue overflowing the container.